The present invention relates generally to a speech signal processing adopted in hearing aids and the like for aiding a weakened auditory sense organ and more particularly to a method of detecting and extending temporal gaps in a speech signal, an apparatus for carrying out the same, and appliances to which the method and the apparatus are applied.
In hearing aids for assisting or aiding the function of the auditory sense organ of hearing-impaired listeners, there have been used mainly analogue type hearing aids in which conventional analogue circuits are employed for processing the amplitude and frequency characteristics of the speech signals. In recent years, however, studies and developments of digital type hearing aids based on digital signal processing technology are energetically carried on. The trend of such studies and developments is reported in detail, for example, in "TREND OF HEARING AIDS UP TO RECENTLY": The Periodical of The Acoustical Society of Japan, Vol. 45, No. 7, (1989), pp. 549-555, and other literatures. The speech signal processing adopted in the digital hearing aid is performed with the aid of a digital signal processor (DSP).
The content of the digital processing to this end is described in the form of a program and stored in a memory. For this reason, alteration or modification of the content of the processing is much facilitated when compared with the conventional analogue type hearing aid, because it can be accomplished simply by altering the program stored in the memory. To say in another way, the digital type hearing aids can easily be adjusted so as to maximize or optimize the clearness of speech to the individual hearing-impaired listeners. In order that the digital type hearing aids replace the analogue type hearing aids, it is a prerequisite that all processings involved be completed with the shortest possible time lag that can not be perceived by the listener or user.
With the speech signal processing adopted in the hearing aids, it is intended to make up for degradation in the frequency resolution, temporal resolution, spectrum discrimination, sound image synthesization and the like abilities of the people with hearing loss. The processings for these purposes are discussed in detail, for example, in "DIGITAL HEARING AID HAVING SPEECH FEATURE EXTRACTING FUNCTION": The Periodical of The Acoustical Society of Japan, Vol, 43, No. 5, (1987), pp. 356-361. Among them, a method of extending temporal gaps (quiescent gap) in a speech signal can be considered as one of the processings to make up for degradation in the temporal resolution capability of the auditory sense organ. According to this temporal gap extending method, temporal gaps or quiescent intervals of a very short duration are inserted between vowels and consonants in a speech signal with inter-word temporal gaps or quiescent intervals being extended for suppressing a temporal masking phenomenon elucidated below. This method is certainly effective for mitigating the temporal masking phenomenon for the hearing-impaired listeners and at the same time for protecting the user against deterioration of the ability for audibly understanding or following the speech.
In case of the hearing aids for the sensory-neural (hearing) impairments who are found in many of the aged persons, it is desirable not only to process the speech frequency characteristic for aiding the frequency resolution capability of the user but also to perform simultaneously the processing for compensating for deterioration in the temporal resolution capability in order to enhance the clearness of speech by processing the speech signal. As a concrete example of the deterioration in the temporal resolution capability, there may be mentioned a so-called "temporal masking" phenomenon that a feeble sound component which immediately follows a strong sound component can not auditorily be discerned due to the masking effect of the latter. Under the circumstances, the hearing impairents often fall into such uncomfortable situation that "although the sound can certainly be sensed, content of the speech can not be understood". This phenomenon is discussed in detail, for example, in "INFLUENCE OF TEMPORAL MASKING TO PERCEPTION OF VOICELESS SOUNDS OF PLOSIVE": Technical Studies Reports of The Institute of Electronics, Information and Communication Engineers of Japan (SP90-97) and other literatures.
In the studies of the technology concerning the detection and extension of the temporal gaps (quiescent intervals)in association with the application to hearing aids, there have heretofore been adopted a method of examining or analyzing the waveform of speech signals display on a cathode ray tube and a method of processing only the speech data of high S/N ratio simply by using a threshold value in the experiments at the level of the laboratory studies.
In order to perform the processing for extending the temporal gaps, it is required to detect the temporal gaps in the speech waves in time domain. In this conjunction, it is noted that the speech waveform lacks steadiness under the influence of background noise or other factors. Consequently, there arises for the detection of the temporal gaps a difficulty that such algorithms and parameters have to be adopted which are less susceptible to the influence of variations in the speech level brought about by noise components.
However, the methods known heretofore are not in the position to extend the temporal gaps dynamically on a real-time basis in dependence on the input speech signal, even though it can provide a simple procedure for detecting the temporal gaps. Such being the circumstance, it is safe to say that there has been proposed no practical means for solving the problems mentioned above which are encountered in the use of the hearing aids.